Polymeric adhesives and paper coatings are used in many disposable packaging applications. Numerous adhesives and glossy coatings are used in the packaging of products such as salt, sugar, tea, coffee and bottle labels, etc. All of these products, and numerous other packaging materials end up for the most part in municipal solid waste (MSW) streams in landfills. Paper and paperboard represent a significant component (.about.35% by volume) of the MSW stream and efforts are underway to recycle certain streams and compost others. These largely cellulosic packaging materials should be compatible with composting or paper recycling operations.
With the rising cost of virgin fiber and the increased demand for wastepaper, the pressure is on to re-use more and more contaminated wastepaper. As a result, contaminant removal, which is essential to convert wastepaper into a reusable fiber, is one of the most important factors influencing the economics of the recycling operation, since this has a direct bearing on the yield of reusable fiber from wastepaper and its total cost. Old newsprint (ONP) is the most abundant used paper fiber source, and is most commonly used for the production of recycled paper. Efficient removal of the ink from ONP can be accomplished only by incorporating about 25 to 40% of old magazine (OMG). The OMG contains clays and mineral particles that facilitate the removal of the ink by a flotation de-inking process. The introduction of OMG also improves fiber strength and brightness levels of the recycled fiber. On the other hand, the incorporation of OMG in the recycling process introduces polymer residues from the adhesives and coatings used to manufacture the magazines.
To benefit the environment, adhesives and other polymeric resins used in paper and paperboard applications should be repulpable and not interfere with the recycling process. In addition, they should be biodegradable and have the required cost and performance characteristics to compete effectively in the market place.
Various natural adhesives (starches, dextrins, etc.) and derivatives of natural products which are biodegradable and have adhesive properties, such as carboxymethyl cellulose, amylose from starch, and casein from milk find uses in adhesive applications. Natural adhesives are used in packaging applications, but they continue to be displaced by synthetics primarily due to performance. Although they are biodegradable and compostable, these natural adhesives cause a problem in paper recycling because they are water soluble, and thus are concentrated in the closed-system water loop of the repulping process where they build up in the initial section of the dryer and on the dryer felts.
With the growing trend of mills re-using their process water, it is becoming as important to effectively remove all contaminants from the pulp flow as it is to remove them totally from the water system in an effort to prevent the accumulation of colloidal impurities. The preferred approach to achieve this requirement is to separate the contaminants at the earliest possible step in the process, but the inherent sticky nature of currently used hot melts and pressure-sensitive adhesive products makes this very difficult. The reduction of water consumption (zero-discharge) with closed water recirculation systems causes reagglomeration of dispersed adhesives resulting in deposits known as "stickiest" on dryer walls and on the polyester `wire`, i.e. the felt on which the recycled paper is deposited. This occurs at very high speeds, and once adhesive residues begin to deposit, build-up occurs exponentially leading to costly mill shut downs.
The residues from adhesives and other polymeric materials currently used in glossy paper coatings, sizing agents, toner particles, etc., which lead to the formation of "stickiest", can have a major impact on the smooth operation and the economics of a paper recycling process. Currently, centrifugal cleaning and fine screening are regarded as the best systems for stickies removal, but these are costly and inefficient.
The commercially available adhesives which are characterized as being repulpable are generally water soluble synthetic adhesives which still cause stickies problems in closed loop recycling mills. Therefore, there is still a need for repulpable adhesives and coatings that match the performance and cost of the predominantly synthetic products now being used. A truly `repulpable` polymer is a polymer which does not persist as "stickies" in a paper recycling process, but which can be quantitatively removed from the process using conventional equipment found in a paper recycling mill.